FIG. 1 a shows a typical ESD protection scheme for input or output (I/O) circuit 10 on a semiconductor chip. The scheme uses a structure in which FO pad 12 to the chip is clamped by protection diodes 14a and 14b, one to the power supply and one to the ground. Protection diodes must have electrical characteristics that provide sufficient ESD protection while not degrading chip performance or leakage characteristics. Key parameters include diode series resistance, subthreshold leakage, and reverse leakage.
The effect of diode series resistance on ESD performance is illustrated in FIG. 2 and discussed in a paper "Scaling, Optimization and Design Considerations of Electrostatic Discharge Protection Circuits in CMOS Technology," By S. Voldman and V. Gross, published in the Journal of Electrostatics volume 33, (1994) page 327-356. It is seen that ESD protection declines as resistance increases.
The series resistance characteristic is especially important in a mixed voltage environment (in which a chip operates at a lower voltage than is available on a bus with which it interfaces) because a string of diodes need be used in place of single diode, as illustrated in FIG. 1b. For example, a chip may operate at a Vdd of about 3.2 V while interfacing with a 5 V bus that may apply the 5 V to I/O pad 12. By selecting an appropriate number of diodes in string 16, one can prevent forward biasing and avoid significant diode current from flowing between FO pad 12 and Vdd during normal operation of the chip, and still provide a current path to Vdd for an ESD transient on FO pad 12. However, each diode in string of diodes 16 introduces series resistance in the current path and these resistances add to significantly degrade ESD protection.
The amount of ESD protection depends inversely on the total resistance of the diodes in string 16. For example, if I/O pad 12 is zapped positively with respect to Vdd at 3000 V, a peak current of 2 A may flow through string of diodes 16. The actual voltage between I/O pad 12 and Vdd depends on the IR drop across diodes string 16, so the lower the resistance of string 16, the lower the voltage, and the more ESD protection offered.
FIG. 1c shows prior art diode 14 formed on p- substrate 20 in n-well 22. Diode series resistance is largely determined by the size of diode 14, the resistivity of well 22 in which diode 14 is located, the distance current flows in n-well 22 and the depth of the path, and by the resistance of silicide films or contacts 24 to n+ and p+ diffusions 26 and 28. Thus, a wider diode with a lower well resistivity, a shorter current path, and silicided films and contacts provide a lower diode series resistance.
As noted above, silicide films and contacts are known to substantially lower device series resistance. However, in diodes having silicided contacts, diode forward and leakage characteristics, and the amount of ESD protection, have been found to be degraded compared with those not having silicided contacts. In devices without silicide, the rapid increase of resistance with temperature tends to cause current to spread uniformly across the extent of the device, and away from small defects that shunt the junction, and this contributes to preventing damage to the protection diode from a high current ESD pulse. Conversely, in devices with silicide films and contacts, the low resistance path through the silicide across the device permits current through the diode to be far more localized at the defect region, hence damage can occur in silicided devices at lower voltages.
In addition, silicide can be responsible for penetration defects that increase forward and reverse bias device leakage currents. This leakage arises if the metal silicide in any region of the diode extends close to the junction depth. The problem of silicide caused leakage increases as junction depth decreases or penetration depth increases. Thus, not only do silicide films and contacts permit localization of current at a defect, silicide can also cause the defect. With each generation of semiconductor technology junction depth is scaled down so solutions to shorting problems due to silicide penetration are highly desirable.